The invention relates to a wire brake and more particularly to a wire brake for automatic wedge bonding machines with cutting pliers arranged behind the associated bonding tool (wedge) and movable along the bonding wire for detaching a bonding wire from the open pliers.
In automatic wedge bonding machines, cutting pliers serve for advancing the bonding wire under the wedge or for holding the bonding wire after completion of a wire bridge, i.e., of the second bonding contact for the purpose of detaching the bonding wire from the second bonding contact. An example of such an automatic bonding machine is found in DE 35 19 594 A1, or alternatively in DE 43 37 837 A1. In both cases the bonding wire must be gripped by the cutting pliers with a predetermined force. Then if the pliers are opened, the bonding wire adheres loosely to one of the jaws of the pliers. But should the automatic wedge bonding machine continue operation despite this unavoidable situation, the bonding process would be negatively affected. For example, irregular wire bridges or uneven tail lengths or even faulty bonds would then have to be expected.
In order to avoid this negative effect on the bonding process, it is generally customary to guide the bonding wire by two small felt pads, which act as a wire brake. Such wire brakes produce a sufficient force that reliably releases the bonding wire from the cutting pliers. In sustained operation, however, problems arise in that the felt bulges and individual loose fibers foul the bonding wire. In addition, dirt adhering to the bonding wire collects in the felt and is uniformly distributed on the bonding wire.
Another variant of realization of a wire brake consists in guiding the bonding wire to the cutting pliers in a Teflon tube. There use is made of the fact that the adhesive force between the bonding wire and the Teflon tube is greater than that between the bonding wire and one of the open jaws of the pliers. Although fouling of the bonding wire by the Teflon tube need not be expected, the undefined braking force must be considered a special disadvantage of this variant. This is the case especially with rapid movements of the bonding wire in the Teflon tube.
Another braking device has been disclosed in U.S. Pat. No. 2,595,270. In particular, a filament brake for yarn, in which the filament is guided through a row of pins, is described here. These pins are fastened on a base and project from it V-shaped alternately with an angle of opening of 10xc2x0. Thus the filament guided through the pins is alternately laterally deflected, so that a braking force is exerted on it. The more deeply the filament dips into the pins, the greater the magnitude of lateral deflection.
Here use is made of the fact that with increasing embrace of the pins, an increase of the braking force exerted on the filament develops.
However, such an accomplishment cannot be transferred to a brake for a bonding wire, since the bonding wire must in no way be permanently deformed mechanically upon feed to the wedge. This risk exists in the accomplishment according to U.S. Pat. No. 2,595,270, where excessive frictional forces are exerted simultaneously on the bonding wire.
The object of the invention therefore is to procure a wire brake in which the disadvantages of the prior art are avoided and with which it is possible to vary the required braking force.
In a wire brake of the type mentioned at the beginning, this object is accomplished in that the wire brake has guiding members that consist of a plurality of deflectors applicable to the bonding wire and has two planes between which the bonding wire, for producing a braking force, can be laterally deflected by a predetermined amount, in that a central deflector deflecting the bonding wire is provided which projects between the two planes for guidance of the bonding wire and is displaceable transverse to the longitudinal extension of the bonding wire, the bonding wire being lifted from the planes by the central deflector.
Production of a bending force on the bonding wire allows the required braking force to be predetermined in exactly defined fashion. In particular, it is of advantage here that the predetermined braking force is independent of the rate at which the bonding wire moves through the wire brake. This means that there will be no negative effect on bonding parameters.
In a preferred refinement of the invention the magnitude of lateral deflection is adjustable.
In an additional embodiment of the invention, a first and a second deflector, which are positioned at some distance apart, are provided opposite the central deflector, and the bonding wire is capable of bearing upon the first and second deflectors.
In order to prevent the guide surfaces themselves from exerting an effect on the braking force, the guide surfaces of the deflectors on which the bonding wire rides have a slightly arched or spherical surface in each instance.
In a preferred embodiment, the guide surfaces of the deflectors are polished.
In addition, it is of advantage if the first and second deflectors are aligned in such a way that the respective normal surface runs perpendicular to the bonding wire. In addition, the bonding wire in the inlet and outlet of the wire brake should be applied tangential to the planes. These measures ensure as small as possible a frictional force caused by the wire brake, so that the magnitude of the braking force is determined chiefly by the bending force.